This invention relates in general to electrostatographic imaging systems, and, in particular, to improved developer materials and their use.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well-known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,69l, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely-divided electroscopic material referred to in the art as "toner." The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
Many methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552 is known as "cascade" development. In this method, developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the electrostatic latent image-bearing surface. The composition of the toner particles is so chosen as to have a triboelectric polarity opposite that of the carrier particles. In order to develop a negatively charged electrostatic latent image, an electroscopic powder and carrier combination should be selected in which the powder is triboelectrically positive in relation to the carrier. Conversely, to develop a positively charged electrostatic latent image, the electroscopic powder and carrier should be selected in which the powder is triboelectrically negative in relation to the carrier. This triboelectric relationship between the powder and carrier depends on their relative positions in a triboelectric series in which the materials are arranged in such a way that each material is charged with a positive electrical charge when contacted with any material below it in the series and with a negative electrical charge when contacted with any material above it in the series. As the mixture cascades or rolls across the imagebearing surface, the toner particles are electrostatically deposited and secured to the charged portions of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharge background. The carrier particles and unused toner particles are then recycled. This technique is extremely good for the development of line copy images. The cascade development process is the most widely used commercial electrostatographic development technique. A general purpose office copying machine incorporating this technique is described in U.S. Pat. No. 3,099,943.
Another technique for developing electrostatic images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method a developer material containing toner and magnetic carrier particles is carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carriers in a brush-like configuration. This "magnetic brush" is engaged with an electrostatic latent imagebearing surface and the toner particles are drawn from the brush to the electrostatic image by electrostatic attraction. Many other methods such as "touchdown" development as disclosed by C. R. May in U.S. Pat. No. 2,895,847 are known for applying electroscopic particles to the electrostatic latent image to be developed. The development processes as mentioned above, together with numerous variations, are well-known to the art through various patents and publications and through the widespread availability and utilization of electrostatographic imaging equipment.
In automatic electrostatographic equipment, it is conventional to employ an electrostatographic plate in the form of a cylindrical drum which is continuously rotating through a cycle of sequential operations including charging, exposure, developing, transfer and cleaning. The plate is usually charged with corona with positive polarity by means of a corona generating device of the type disclosed by L. W. Walkup in U.S. Pat. No. 2,777,957 which is connected to a suitable source of high potential. After forming a powder image is electrostatically transferred to a support surface by means of a corona generating device such as the corona device mentioned above. In automatic equipment employing a rotating drum, a support surface to which a powdered image is to be transferred is moved through the equipment at the same rate as the periphery of the drum and contacts the drum in the transfer position interposed between the drum surface and the corona generating device. Transfer is effected by the corona generating device which imparts an electrostatic charge to attract the powder image from the drum to the support surface. The polarity of charge required to effect image transfer is dependent upon the visual form of the original copy relative to the reproduction and the electroscopic characteristics of a developing material employed to effect development. For example, where a positive reproduction is to be made of a positive original, it is conventional to employ a positive polarity corona to effect transfer of a negatively charged toner image to the support surface. When a positive reproduction from a negative original is desired, it is conventional to employ a positively charged developing material which is repelled by the charged areas on the plate to the discharge areas thereon to form a positive image which may be transferred by negative polarity corona. In either case, a residual powder image and, occasionally, carrier particles remain on the plate after transfer. Before the plate may be reused for a subsequent cycle, it is necessary that the residual image and carrier particles, if any, be removed to prevent ghost images from forming on subsequent copies. In the positive-to-positive reproduction process described above, the residual developer powder, as well as any carrier particles present, are tightly retained on the plate surface by a phenomenon that is not fully understood but believed caused by an electric charge. The charge is substantially neutralized by means of a corona generating device prior to contact of the residual powder with a cleaning device. The neutralization of the charge enhances the cleaning efficiency of the cleaning device.
Typical electrostatographic cleaning devices include the "web" type cleaning apparatus as disclosed, for example, by W. P. Graff, Jr., et al in U.S. Pat. No. 3,186,838. In the Graff, Jr., et al patent, removal of the residual powder and carrier particles on the plate is effected by rubbing a web of fibrous material against the imaging plate surface. These inexpensive and disposable webs of fibrous material are advanced into pressure and rubbing or wiping contact with the imaging surface and are gradually advanced to present a clean surface to the plate whereby substantially complete removal of the residual powder and carrier particles from the plate is effected.
While ordinarily capable of producing good quality images, conventional developing materials suffer serious deficiencies in certain areas. Some developer materials, though possessing desirable properties, such as proper triboelectric characteristics, are unsuitable because they tend to cake, bridge, and agglomerate during handling and storage. The developing materials must flow freely to facilitate accurate metering and even distribution during the development and developer recycling phases of the electrostatographic process. Adherence of carrier particles to reusable electrostatographic imaging surfaces causes the formation of undesirable scratches on the surfaces during the image transfer and surface cleaning steps. In addition, the triboelectric values of some developer materials fluctuate with changes in relative humidity and, therefore are not desirable for employment in electrostatographic systems, particularly in high-speed automatic machines which require developer materials having stable and predictable triboelectric values. Another factor affecting the stability of the triboelectric properties of developer materials is the susceptibility of developer particles to "toner impaction." When developer particles are employed in automatic machines and recycled through many cycles, the many collisions which occur between the carrier particles and other surfaces in the machine cause the toner particles carried on the surface of the carrier particles to be welded or otherwise forced onto the carrier surfaces. The gradual accumulation of impacted toner material on the surface of the carrier causes a change in the triboelectric value of the carrier and directly contributes to the degradation of copy quality by eventual destruction of the toner carrying capacity of the carrier.
Thus, there is a continuing need for a better developer material for developing electrostatic latent images.